1. Field of the Invention
The present invention relates to turbofan engine noise reduction. Specifically, the present invention relates to reducing turbofan engine noise using innovative exit guide vanes.
2. Description of the Related Art
Fan exit guide vanes in a turbofan engine are used to remove the swirl in the flow downstream of the fan rotor and increase the thrust and efficiency of the engine. However, the exit guide vanes are typically located close enough to the fan rotor that their interaction with the rotor wake perturbations generates high levels of tone and broadband noise. Significant efforts have been directed toward the reduction of this unwanted noise.
The most common technology used to reduce turbofan engine noise involves acoustic treatment, or liners, along the walls of the fan duct, or nacelle, to absorb the interaction noise once it has been generated. However, proper design of liners of this type requires a detailed understanding of sound propagation though the highly complex engine environment.
A more recent approach to reduce turbofan engine noise involves increasing the spacing between the fan and the exit guide vanes so as to mitigate the impinging flow perturbations. This approach attempts to reduce noise at the source, rather than abate the noise elsewhere. Unfortunately, this approach generally leads to an increase in engine length and a corresponding increase in engine weight. Increasing engine weight is highly undesirable due to the associated added fuel burn penalty.
Another source modification approach to reduce turbofan engine noise involves modifying the fan rotor blade and or exit guide vane geometry in a manner that mitigates the strength of the interaction between the two. These modifications include introduction of sweep and/or lean into the fan rotor blades and/or exit guide vanes. However, this approach often requires a delicate balance to be struck between structural integrity of the engine components and the aerodynamic performance of the engine.
The use of resonance or noise reducing chambers or cavities is a known method for reducing noise in various instances. For example, automobiles commonly use combinations of resonators and bulk absorbing materials in mufflers to reduce exhaust noise. One type of resonator, known as a Helmholtz resonator, provides a frequency selective way to modify a complex noise spectrum, where the targeted frequency is a function of the chamber volume, chamber entrance geometry, and the speed of sound. Furthermore, the use of sound absorbing materials inside resonators is also known to improve the noise reduction benefits. However, these methods have heretofore not been applied within the structure of the exit guide vanes to achieve noise reduction in turbofan engines.
Lastly, U.S. Pat. No. 6,139,259, issued to Ho et al., discloses a low noise permeable airfoil. According to the disclosure, this stator airfoil includes a plurality of cross channels that extend through the airfoil. The cross channels bleed fluid between the two sides of the airfoil to reduce the fluctuating pressure differential between the two sides to reduce noise. Unfortunately, this approach degrades the aerodynamic performance of the airfoil, namely, the ability of the airfoil to turn the flow and remove the swirl.
There remains a need for improved turbofan engine noise reduction technologies. Thus, it would be advantageous to provide a new technology for reducing turbofan engine noise without significantly reducing engine performance. Specifically, it would be advantageous to reduce turbofan engine noise using noise resonant chambers inside the fan exit guide vanes.